Preheat the oven to 180C/350F/gas mark 4
Use a 23cm/9 inch square cake tin.
Grease and line the cake tin. Dust the raisins and nuts with a little flour. Sift half the remaining flour with the spices and carob and the other half with the baking powder. Set aside.
Cream the sugars and the butter until light and fluffy. Beat in the mashed potato and the sour cream. Mix in the spiced flour and blend well. Then stir in the milk and add the rest of the flour. Stir in the nuts, raisins and lemon rind. Pour the cake batter into the prepared tin and bake in the preheated oven for one hour, or until a warm knife inserted into the centre comes out clean. Leave to cool in the tin for ten minutes, then turn out to cool on a wire rack.